Pediatric fluid and electrolyte management

👶 Little Bodies, Big Differences

• Higher Body Surface Area (BSA) to Weight Ratio: ↑ insensible water losses.
• Higher Metabolic Rate: ↑ fluid turnover and oxygen demand.
• Immature Kidneys:
  • Lower Glomerular Filtration Rate (GFR).
  • Poor urine concentrating ability; can't conserve water efficiently.
• Limited Glycogen Stores: High risk for hypoglycemia with fasting/stress.
• Body Composition: Higher % of Total Body Water (TBW), especially extracellular fluid (ECF).

⭐ Infants' larger ECF volume (as a % of TBW) means they lose a greater proportion of their total fluid with vomiting or diarrhea, leading to rapid dehydration.

💧 Management - The Fluid Formula

The Holliday-Segar method calculates daily maintenance fluid needs based on weight.

• First 10 kg: $100 \text{ mL/kg/day}$
• Next 10 kg (11-20 kg): $50 \text{ mL/kg/day}$
• Each kg > 20 kg: $20 \text{ mL/kg/day}$

📌 "4-2-1 Rule" for Hourly Rate:

• First 10 kg: $4 \text{ mL/kg/hr}$
• Next 10 kg: $2 \text{ mL/kg/hr}$
• Each kg > 20 kg: $1 \text{ mL/kg/hr}$

⭐ Isotonic fluids (e.g., D5 NS, LR) are now preferred over hypotonic fluids for maintenance in most hospitalized children to reduce the risk of iatrogenic hyponatremia from non-osmotic ADH secretion.

🔧 Management - Fixing the Deficit

• 1. Calculate Fluid Deficit (mL):

  • $ \text{Deficit} = % \text{Dehydration} \times \text{Weight (kg)} \times \mathbf{10} $
  • e.g., 10% dehydrated 10 kg child = 1000 mL deficit.

• 2. Initial Resuscitation (if needed):

  • For moderate-severe dehydration/shock.
  • Bolus: 20 mL/kg isotonic crystalloid (NS/LR) over 15-30 min.
  • May repeat 2-3 times.

• 3. Plan Deficit Replacement:

  • Subtract bolus from total deficit.
  • Replace remaining deficit + maintenance fluids over 24-48 hrs.
  • 📌 Common plan: 50% of deficit in first 8 hrs, remaining 50% over next 16 hrs.

⭐ Slow is Safe for Sodium!

• Hyponatremia: Correct Na⁺ ≤ 8-12 mEq/L per 24h (prevents ODS).
• Hypernatremia: Correct Na⁺ ≤ 0.5 mEq/L/hr (prevents cerebral edema).

🎢 Pathophysiology - Electrolyte Rollercoaster

• Sodium ($Na^+$):
  • Hyponatremia (<135 mEq/L): Often post-op (↑ADH) or from hypotonic fluids. Risk of cerebral edema, seizures.
  • Hypernatremia (>145 mEq/L): Dehydration (diarrhea, fever). Risk of intracranial hemorrhage during rehydration.
• Potassium ($K^+$):
  • Hypokalemia (<3.5 mEq/L): GI losses (NG suction, vomiting). Causes ileus, ECG changes (U waves).
  • Hyperkalemia (>5.5 mEq/L): Renal failure, cell lysis. ECG changes (peaked T waves).

⭐ The most common electrolyte abnormality in hospitalized children is iatrogenic hyponatremia due to hypotonic fluid administration, leading to seizures.

⚡ Biggest Takeaways

• Calculate maintenance fluids with the 4-2-1 rule: 4 mL/kg/hr (first 10 kg), 2 (next 10), 1 (rest).
• Use isotonic fluids (D5 NS/LR) for maintenance to prevent iatrogenic hyponatremia, especially post-op (↑ADH).
• Resuscitate with rapid 20 mL/kg boluses of isotonic crystalloid (NS or LR) for shock.
• Tachycardia is the earliest sign of dehydration; also check for sunken fontanelles and poor turgor.
• Add dextrose to maintenance fluids to prevent hypoglycemia in infants with low glycogen stores.